Glutamate is an important excitatory neurotransmitter in the mammalian central nervous system. Glutamate synaptic responses in the central nervous system (CNS) are mediated via activation of two families of receptors; ligand-gated cation channels termed ionotropic glutamate receptors, and G-protein-coupled receptors known as metabotropic glutamate receptors (mGluRs). Thus far, eight mGluR subtypes, together with splice variants, have been cloned and characterized in functional studies (Schoepp et al. Neuropharmacology, 1999, 38, 1431-1476). The eight mGluRs are grouped into three classes based on structural homology, pharmacology and signal transduction mechanisms.
Group I receptors (mGluR1 and mGluR5) couple through Gq/11 proteins to the activation of phospholipase C (PLC) resulting in phosphoinositide (PI) hydrolysis, the release of calcium from intracellular stores. While group II (mGluR2 and mGluR3) and III (mGluR4, mGluR6 mGluR7 and mGluR8) are negatively coupled to adenyl cyclase (AC) through G1/Go proteins thereby inhibiting cyclic AMP (cAMP) formation (Francesconi and Duvoisin, 1998).
Glutamate and Pain
Chronic pain is an area of high unmet medical need. Current therapies are not adequate and chronic pain is often refractory to most commonly used analgesics, including opioids. Glutamate plays a major role in nociceptive processing. Glutamate receptors, including mGluRs, are expressed in relevant areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission.
Chronic pain may be due to tissue injury and diseases (inflammatory pain) or of the central and peripheral nervous system (neuropathic pain) and is associated with severe chronic sensory disturbances characterized by spontaneous pain, hyperalgesia (exaggerated responsiveness to painful stimuli) and allodynia (wrong perception of non noxious stimuli as painful). Prevalent symptoms in human patients include cold hyperalgesia, mechanical allodynia and less commonly, heat hyperalgesia.
Chronic pain is a true disease. It is believed to be a result of the plasticity at synapses in nociceptive processing centers, a phenomenon referred to as “central sensitization” which consists of increased excitability of spinal cord dorsal horn neurons. Glutamate receptors have been identified for their key role in central sensitization. Plasticity at synapses involved in nociceptive processing requires activation of ionotropic glutamate receptors NMDA and this plasticity is modulated by mGluRs including mGluR1. NMDA receptor antagonists have been tested in experimental therapies for the prevention and treatment of persistent pain following injury. However there are significant undesiderable side effects associated with the use of NMDA antagonists due largely to the critical role of those receptors in normal excitatory synaptic transmission throughout the nervous system. These side effects include pyschosis, hyperactivity, fatigue, dizziness, and in the case of higher levels of NMDA antagonists, amnesia and neuronal toxicity. Drugs designed to target mGluRs responsible for persistent alterations in nociception such as antagonists at mGluR1 might have reduced effects on excitatory transmission since their role of modulators of NMDA receptor-dependent plasticity in the dorsal horn, while effectively modifying the abnormal elevation of transmission thought to underlie persistent pain states. Thus mGluR antagonists might perform well clinically in chronic pain states without the side effects inherent to NMDA receptor antagonists.
mGluR1 and Pain
A number of behavioral (Fisher et al. Neuroreport, 1998, 20, 1169-1172; Fundytus et al. Neuroreport, 1998, 9, 731-735; Bhave et al. Nature Neurosci., 2001, 4, 417-423; Dolan et al. Neuropharmacology, 2002, 43, 319-326; Dolan et al. Pain, 2003, 106, 501-512) and electrophysiological (Young et al. Neuropharmacology, 1994, 33, 141-144; and Young et al. Brain Res., 1997, 777, 161-169) studies have demonstrated a specific role for Group I mGluRs, and in particular mGluR1 receptors, in nociceptive processing in the CNS, including mechanisms of hyperalgesia and inflammation. In the spinal cord, mGluR1 appears to be localized primarily on postsynaptic elements throughout the dorsal and ventral horns. (Neugebauer, Trends Neurosci., 2001, 24, 550-552). The intrinsic activation of spinal mGluR1 in chronic nociception has been demonstrated using antagonists, antibodies and antisense oligonucleotides. Intrathecal administration of an mGluR1 antagonist produced antinociceptive effects in the second phase of formalin-induced nociceptive behavior (Neugebauer, Trends Neurosci., 2001, 24, 550-552). Behavioral studies have also addressed the role of spinal mGluR1 receptors in the spinal injury and ligation models of neuropathic pain. Expression of mGluR1 is increased in rats following spinal cord injury and this may mediate the chronic central pain induced by the injury (Mills and Hulsebosch, Neurosci. Lett., 2002, 319, 59-62). Knockdown of spinal mGluR1 by intrathecal infusion of antisense oligonucleotides attenuated cold hyperalgesia and mechanical allodynia in neuropathic rats (Fundytus et al. Br. J. Pharmacol., 2001, 132, 354-367; and Fundytus et al. Pharmacol. Biochem. Behav., 2002, 73, 401-410). Additionally, spinal administration of anti-mGluR1 IgG antibodies reduced cold hyperalgesia, but not mechanical allodynia, in neuropathic rats (Fundytus et al. Neuroreport, 1998, 9, 731-735). The critical role of spinal mGluR1 receptors in pain-related central sensitization is emphasized at the single cell level by electrophysiological in vivo studies in anesthetized animals. Intraspinal administration of an mGluR1 antagonist inhibited the responses of primate spinothalamic tract neurons to brief noxious, but not innocuous, mechanical cutaneous stimuli, as well as central sensitization in the capsaicin pain model (Neugebauer et al. J. Neurophysiol., 1999, 82, 272-282). In rats with knocked down mGluR1 expression, the responses of multireceptive dorsal horn neurons to noxious input evoked by repeated topical applications of the C-fiber irritant mustard oil were significantly reduced compared to control neurons; the responses to innocuous cutaneous stimuli were not significantly different (Young et al. J. Neurosci., 1998, 18, 10180-10188).